Control of AC motor/generators, such as three-phase permanent magnet synchronous electric motors (electric machines) is accomplished using a three-phase pulsewidth-modulated (PWM) inverter. A PWM inverter can be controlled in several different operating modes, including, e.g., a linear modulation mode and an over-modulation mode. One example of a linear modulation mode of operation is a space vector PWM (SVPWM) mode with linear modulation, e.g., up to 90% of a six-step operation, and one example of an over-modulation mode is a full six-step mode.
Magnitude of the output voltage from an inverter at its fundamental synchronous frequency reaches its maximum only when the inverter operates in the over-modulation mode. Due to this voltage magnitude characteristic, operation in the over-modulation mode can increase torque capability of an electric machine in a field-weakening region as compared to operation in a linear modulation mode. This is due to voltage magnitude being a major limiting factor of the torque capability of the electric machine.